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The former ANC group experimented on different designs for a lime feeder, including a column model, a conical vessel, a funnel - column and lastly an inverted traffic cone model. The most effective design was the inverted cone because it supplied saturated limewater at a pH between 11 and 12 for about 18 hours without having to be unclogged. However, the main problem with this setup is that inverted cones are extremely difficult and expensive to construct, install and maintain. Every other model also had its problems and drawbacks.
For example, the Column model succeeded in keeping the lime suspended for a few hours but the water began to flow in a preferential path after the lime settled on the bottom. For the conical column, the mixing at the bottom of the vessel proved to be inefficient in keeping all the lime in a suspended state. On the other hand, the funnel-column apparatus worked well for 20 hours but only because it was unclogged periodically, which would not possible in a real-time set up. As a result, the inverted cone model that gave a 12 hour success run, without the above difficulties was selected as the best alternative among them. However, due to the difficulty in construction, installation, and maintenance, the ANC team was assigned the task of searching for a simpler solution for the lime feeder design.
ALKALINITY IN HONDURAN WATER
The table below shows actual measures of pH and alkalinity in AguaClara treatment plants in Honduras. To have high accuracy in the lime feeder design, the team will simulate the conditions of raw water in the lab.
!HONDURAN ALKALINITY|thumbnail,align=centre!
PROPERTIES OF LIME
The results demonstrate a decrease on pH during teh treatment process, it is strongly visible on Cuatro Comunidades and Tamara plants.
Souce: Honduras water reports, 2009
EXPERIMENTAL METHODS AND RESULTS
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The lime feeder water flow rate required for the experiment was calculated with Mathcad. The analysis was made according with the dosage of lime, the operation time, water velocities, water pH and other variables, most of them were taken from the previous team report. The mathcad file with the results is as below.
<LINK> of mathCAD file
ConcurrentlyOn this stage, the team also continued to search for other information including the cost of distilled water, lime properties (solubility with temperature), and Honduras' water regulation.
PROPERTIES OF LIME
STAGE TWO (Experimental Trials 1, 2 and 3)
The first experimental task was to check the feasibility of a fluidized lime bed suspended in a tall column. This is the simplest model that can be used as a lime feeder. The aim was to find an optimum upflow velocity that would mix the lime uniformly and keep it suspended but not cause any without causing a great amount of lime to go out with the effluent. Upflow velocity can be calculated by dividing the flow rate by the cross sectional area of the column. Since the area is constant, upflow velocity is directly proportional to the flow rate.
The experimental set up can be seen in the figure below.
Figure - 1
Trial 1:
This experiment consisted of measuring the upflow velocity with different flow rates and different quantities of lime, while always maintaining a suspended column of lime. This was an initial experiment and hence most of the quantities were taken through approximation. The details of the exact quantities of lime and flow rates as well as the results obtained can be found in the attached file called "ANC-Initial Experiment using Column".
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The optimum flow rate which could keep the lime suspended without excessive lime coming out (as observed visually) was found to be about 85ml/min.
The figure above shows the apparatus used for the first trial, it consists in a tube conected with a water source regulated by a pump. The lime was added manually at the top.
Figure - 1 
Trial 2 and 3:
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The results of the experiment are on an attached excel file.
<LINK> Trial 2 and 3 on Column experiment
Figure - 1
Identified ProblemsThe problems identified with these trials were:
1) The arrangement of the pH probe was not stable so the pH data was not accurate.
2) The simple column design was not adequate in giving any range in terms of flow rates that could fulfill the objectives of maintaining a good suspension and maintaining a stable pH in the resulting water solutionfor 24 hours on the effluent.
3) The vertical column alone was not sufficient enough to store and adequate the required amount of lime to run the experiment for 24 hours.
These problems prompted for a modification in the design of the column leading to the next stage of experiments.
By the end of Stage stage two, thus, changes were made (as per following Professor Monroe's suggestions) in the design of the water outlet in order to ensure an adequate pH measure. It included putting the pH probe in a vertical tube and making a curved outlet for the effluent. With these changes, the team apparatus could get a more stable pH measure. The A figure explaining this description is shown below.
FIGURE - 2 to be added (pH probe arrangement)
On teh second stage, the The team also worked on the relationship between the ratio of flow rate between the lime feeder and the plant with the alkalinity provided by the effluent lime. That information was useful to make changes on the edit MathCAD calculations and to find determine the changes in the Ph at the plant with respect to changes in this flow rate ratio (fraction of water flow rate entering the lime feeder from the entrance tank).
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In order to overcome the difficulties faced at the end of stage two, the team considered a new design, which consists of a diagonal column attached at the top of the vertical column. The design would help the saturated lime-water solution stay inside the apparatus, while still having the needed amount go outconcentration at the exit. Since the velocity in the slanted tube is at an affected by the angle, its vertical velocity component is less lower than the upflow velocity of the primary column. This decreased velocity allows more lime to settle back into the column and thus prevent unnecessary lime loss. Thus the primary column would be used as a storage vessel for the suspended lime bed while the slanted tube above it would allow for a more uniform saturated lime mixture.
The dimensions of apparatus will be are determined according to MathCad calculations; the relevant criteria being the length of both columns. Below is the link for the mathCAD file:
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The team worked on calculations using the following assumptions for simplification:
- Dfrac = 3: It is assumed that when When elementary lime particles coagulate, the density of the larger mass stays the same as that of the original particles. This is unlike what happens in flocs, that have a Dfrac of 2.3.
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Results of MathCAD calculations: Theoretically, the maximum length of the tube should not exceed 1.5m. In actuality, the lime Lime particles will have a larger density much larger than the flocs, which means their settling velocities will be much higher than the assumed 10m/day. Also, we do not need ALL the it is not necesary that ALL lime particles to settle down - some amount (a particular amount, not determined yet determined) will have to eventually fall out of the lime feeder to solve the acidity problem. Consequently, the length of the tube we needed will need will actually be much less lower than 1.5m.
The length of needed for the pipe needed in order to obtain a fully developed laminar flow of water in the pipe, 'Le', was also calculated and determined to be about 10cm as per in 10cm with the given (above) conditions. This was just is required to verify whether or not there was is a fully developed parabolic profile by at the end of the pipe or not.The . In conclussion, the length of the tube must be greater than Le.
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Figure - 3 : the picture of new apparatus will be taken this week and put up (since we will get the constructed apparatus only by 10/27/09).
The final target for this duration is 24 hours so that the operator at AguaClara plants in Honduras need only refill the lime feeder once a day. For this trial, distilled water will be used instead of tap water to better simulate Honduran water. In the picture below (figure 4), the ANC Control team can be seen carrying the distilled water tank on to the platform where the experiment is to be set up.
Figure - 4 
FUTURE TASKS
- Is is required to develop several trials with the new enmseble. The team will determine the amount of lime required to maintain a pH of 12 for 24 hours. In addition the team will evaluate the performance of the new emsemble, if it is necessary, the length will be calculated again.
- Simulate Honuran water conditions. Once the team will have the design working well with distilled water, a new experiment will be required to analyze the performance with a lower pH concentration.
- The team is also working on the calculation of required changes according with the natural cariation of flow rate at the inlet tank in the plant.
